In many manufacturing processes, particularly in the food, drug and cosmetics industries, there is a need to put a coating on, or otherwise modify the surface of host substances (e.g. particles) for a particular purpose. Such coatings can be applied in conventional fluidized beds which typically have porous bottoms with gas flowing upwardly therethrough. When the drag force of the gas on the particles is exactly balanced by the gravity force, the particles lift and form a fluidized bed. The velocity at which the bed becomes fluidized is called the minimum fluidization velocity. In rotating fluidized beds, one can fluidize very fine particles which cannot be fluidized in conventional beds. Another major advantage of rotating vs. conventional fluidized beds is that higher amounts of gas can be passed through in a rotating bed as the effective force on the particles is greater due to the high centrifugal force.
Coating of powders can be accomplished by either "wet" or "dry" methods. Typically, dry methods to synthesize composite materials use some grinding systems. In such methods, core particles are coated using fine particles by subjecting the mixture to large shearing and compressive stress or high impact forces. Various scientists have studied dry coating methods to synthesize composite materials using a high shear mill called Mechnofusion. Some have have analyzed the dry coating process using a dry impaction blending method referred to as a Hybridizer. Others have developed a horizontal high shear mill (Theta composer) for the particle dry coating. In all of these devices, the high shear stress or impaction force may determine the depth to which the guest particles are embedded into the surface of the host particles. The local temperature rise due to these strong forces may also play a role in determining the coating strength.
Granulators and coaters must be distinguished. A coater is used in coating of relatively fine (guest) particles onto relatively large core (host) particles to create new material with new functionality or to improve the characteristics of known materials. For instance, the guests may be 1-5% in size compared to the hosts. By contrast, a granulator does not involve the addition of a guest particle, but the enlarging and rounding of small, irregularly-shaped particles. The granulated particle may grow to several times the size it was in its ungranulated state.
Wet granulation processes typically use a binder such as water, sugar solution, starch, various polymers, etc. The "wet" methods used for coating, require spraying a solvent or a suspension onto the powdery particles. Dry methods are thus preferred because they do not require the use of solvents and are therefore environmentally friendlier.
A device for coating powders is disclosed in Japanese Patent Laid-Open Publication No. 7-299348. The coating device includes a rotary member having a plurality of air supply holes, a plurality of agitating blades and a container having a powder supply hole. The powder is supplied into the container through the powder supply hole positioned above the rotary member toward the rotary member. A fluidization (in hybridization, there is no fluidization; rather pneumatic transport) is made in such a manner that the rotary member and the agitating blades are rotated, with air supply being made through the air supply holes and the powder is supplied from the powder supply hole.
It is therefore an object of the invention to provide an efficient and economical rotating fluidized bed coater which:
(a) works on lower levels of normal and shear stresses as compared to a hybridizer and mechanofusion;
(b) produces less heat hence has lower operating temperature, yet, by changing the temperature of the incoming gas stream, the process temperature may be controlled;
(c) allows the gas stream to carry another substance, for example moisture, mist of another liquid, or even another dispersed powder material, thus allowing better possibility for creating advanced composite powders;
(d) can be easily operated as a continuous operating device, thus does not have the limitation of a "batch" process;
(e) because of some of the above advantages may be more easily applicable to pharmaceutical and other "delicate" applications;
(f) allows air, inert gas or reactive gas (including reducing or oxidizing type), to be the fluidizing medium, depending on the need and/or application;
(g) is less likely to cause contamination of the powder material as compared to devices with higher stresses (e.g. hybridizer and mechanofusion); and
(h) provides improved material properties such as flowability, dispersability, coloring, flavor/taste, etc and improved chemical reaction functions; as well as
(i) allows for the creating of discrete, as well as continuous or film type coatings.